OS News reports Microsoft has had to define a separate application class, aside from Metro and desktop applications, to make Windows 8 compatible with third party web browsers:
Web browsers and Windows have a troublesome history. Internet Explorer was supposedly tied so deep into Windows, it couldn't be removed, according to Bill Gates. This put the competition - Firefox, especially - at a severe disadvantage. So much so, even, that the European Union forced Microsoft to implement that silly browser ballot box thing.
In trying to bring the restricted iOS-like environment from Windows Phone 7 to the desktop, Microsoft ran a risk of pissing off the European Union again. Since the Metro environment imposes numerous restrictions that would hinder the development of a decent browser, Microsoft has created a new application class, called "Metro style enabled desktop browser".
This type of browser will work in both the traditional desktop, with a traditional interface, as well in Metro, with a fancy modern Metro interface. The user then gets to decide which variant of the browser he uses - similar to how Internet Explorer 10 currently works in Windows 8.